The present invention relates to an ultrasonic transducer employed for ultrasonic diagnosis systems.
Ceramics of the type of zirconium lead titanate (PZT) have heretofore been much used as materials for piezoelectric vibrators in ultrasonic transducers. However, these piezoelectric ceramics (i) have acoustic impedances that are much greater than that of a human body, and require contrivance in regard to acoustic matching layer when they are to be used for diagnosing purposes, (ii) have exremely large dielectric constants and, hence, small piezoelectric voltage constants g, making it difficult to obtain a high voltage when ultrasonic waves are received, and (iii) are not adapted to be curved so as to fit to the shape of a human body. In order to solve these problems, there has been proposed a so-called piezoelectric composite consisting of a combination of a polymer and a piezoelectric material. As an example, there has been reported by Newnham et al. of U.S.A. that a composite material consisting of a polymer in which is buried a PZT pole is effective (Materials Research Bulletin, Vol. 13, pp. 525-536, 1978). There has, in practice, been obtained a composite material which consists of PZT and a polymer such as silicone rubber, epoxy resin, or the like, and which exhibits a small acoustic impedance and a large piezoelectric voltage constant g.
When the ultrasonic waves are to be transmitted and detected using such a piezoelectric composite, it is desired that the polymer and portions of the piezoelectric poles undergo uniform displacement. Generally, however, the polymer is considerably softer than the piezoelectric poles. In practice, it has been clarified that the piezoelectric poles undergo displacement more greatly than the polymer portion. When the ultrasonic waves are to be transmitted, therefore, there develops acoustic noise, i.e., a so-called grating lobe. The grating lobe consists of undesirable ultrasonic waves other than main ultrasonic waves, the grating lobe being emitted in the directions determined by a pitch of piezoelectric pole arrangement to deteriorate the ultrasonic image.